Iso-1-cytochrome c and iso-2-cytochrome c from the yeast Saccharomyces cerevisiae are two of the few proteins of known primary structure from a microorganism which is particularly suitable for experimental genetic studies and for manipulation by recombinant DNA procedures. The isolation of appropriate mutants have been facilitated by enrichment procedures for both forward and reverse mutations. A series of deletions are available, making it possible to conveniently map point mutants and to estimate their positions relative to the iso-1-cytochrome c sequence. The large number of mutants that have been characterized and the selection of procedures permit an unprecedented degree of genetic manipulation of nucleotide sequences by recombination. The DNA sequences of the structural genes of iso-1-cytochrome c (CYC1) and iso-2-cytochrome c (CYC7) as well as portions of the adjacent regions have been determined. Thus, the body of information concerning the iso-1-cytochrome c gene, the large number of defined mutants and the available genetic and biochemical techniques are without parallel for any other eukaryotic gene. This iso-cytochrome c system is being employed for investigating numerous problems in molecular biology and genetics, including the following: DNA sequencing, transcriptional analysis, gene assignment and evolutionary relationship of the two 6 kb regions, COR and ARC, which encompass, respectively, the CYC1 and CYC7 genes; regulation and maturation of the two iso-cytochromes c and the role of heme, catabolite repression and anaerobiosis on the synthesis of CYC1 and CYC7 mRNAs and their corresponding apo- and holo-proteins; DNA alterations of the regions adjacent to the CYC1 locus and their effect on initiation and termination of transcription and initiation of translation; investigation of the overproduction of iso-2-cytochrome c caused by alterations in the CYC7 prefix region and by unlinked mutations; DNA sequencing of missense mutations and structure-function relationship of iso-1-cytochrome c; amino acid inserted by ribosomal suppressors and the efficiencies of suppression; relationship of recombination frequencies and nucleotide alterations; investigation of the action of mutagens and the nucleotide sequences of mutable and immutable sites and of unstable mutations.